In recent years, a shift by-wire system is adopted in vehicle control. For example, a shift by-wire system includes an actuator for changing of a vehicle state. The actuator is electrically controlled with a by-wire control circuit in accordance with a passenger's instructions. According to JP-A-2001-271917, the shift range of an automatic transmission of a vehicle is changed in accordance with a vehicle passenger's instructions in a shift by-wire system.
In this structure, a range position switching unit switches the shift range of an automatic transmission according to a driving current, which is output from a range control unit.
The range control unit sets the driving current in order to steadily switch the shift range of the automatic transmission. The range position switching unit includes a motor that regularly generates driving force sufficient to switch the shift range of the automatic transmission.
By contrast, the driving force required for switching varies according to a switching pattern of the shift range in the automatic transmission. For example, when the shift range of the automatic transmission is present in a P range, rotation of an output shaft of the automatic transmission is mechanically restricted by a lock mechanism. Therefore, when the shift range of the automatic transmission is switched over from the P range to an R range, force is needed for release of the lock mechanism. Consequently, when the shift range is switched over from the P range to the R range, the maximum driving force is required of the motor of the range position switching unit. Hereupon, output of the motor of the range position switching unit is set on the basis of the driving force, which is required when the shift range of the automatic transmission is switched over from the P range to the R range.
However, when the switching pattern of the shift range of the automatic transmission is other than switching-over from the P range to the R range, the shift range can be switched over even when the driving force generated from a motor is small. Thereby, even when the switching pattern of the shift range is other than switching-over from the P range to the R range, the driving current is supplied to the motor to correspond to the maximum driving force, which affords switching-over from the P range to the R range. Consequently, power consumption of the motor increases. In addition, the driving force increases a load applied to the range position switching unit and the automatic transmission.